Need to seal a valve/radiator connection

The connection between the radiator and the valve is leaking steam and water. The nut is as tight as it can possibly be tightened. It looks like there was some sort of sealant applied around the nut, but it has failed.



Is there a sealant that I can apply? Or should I disconnect the radiator and seal the thread as I would with a typical plumbing connection? What should i use on the threads?



This is a one pipe steam heating system. Everything else works perfectly. I only discovered this leak today. Our tenants of the last two years have recently moved and i was going through the apartment fixing things when I realized this radiator was constantly hissing.